Beta-spodumene and beta-spodumene solid solutions are crystalline aluminosilicates of tetragonal structure which form a major component of many important ceramic and glass-ceramic products. Glass compositions stoichiometric of beta-spodumene (Li.sub.2 O . Al.sub.3 . 4SiO.sub.2) or solid solutions thereof with silica (Li.sub.2 O . Al.sub.2 O.sub.3 . nSiO.sub.2, wherein n ranges from about 3.5- 10) exhibit acceptable melting and forming characteristics; yet ceramic products comprising these crystals can be refractory and quite low in thermal expansion.
The crystal chemistry of beta-spodumene solid solutions is described by B. J. Skinner and H. T. Evans, Jr. in Am. J. Sci., Bradley Vol. 258A, pp. 312- 24 (1960). The manufacture of nonporous glass-ceramic articles comprising beta-spodumene solid solution as a principal crystal phase by the controlled crystallization in situ of lithium aluminosilicate glasses is reported by Stookey in U.S. Pat. No. 2,920,971. Other beta-spodumene glass-ceramics are described by Voss et al. in U.S. Pat. No. 3,148,894 and Bruno et al. in U.S. Pat. No. 3,582,371.
The extraction of lithium from mineral beta-spodumene by a hydrogen-for-lithium ion exchange process has been known since at least 1950, being described by Ellestad et al. in U.S. Pat. No. 2,516,109. More recently, Grossman et al. disclosed in U.S. Pat. No. 3,834,981 that lithium may be extracted from unitary ceramic articles comprising beta-spodumene solid solution crystals by the same hydrogen-for-lithium ion exchange. This extraction process is non-destructive, and provides a ceramic product comprising a new crystal phase, termed aluminous keatite, consisting of alumina and silica in proportions corresponding to that of the original composition.
Briefly, the process described in the aforementioned Grossman et al. patent comprises the steps of contacting a selected beta-spodumene-containing ceramic article with a strong mineral acid to replace at least some of the lithium present in the crystals with hydrogen. Thereafter the ceramic article is heated to provide aluminous keatite crystals through the removal of at least some of the water of crystallization therefrom.
Either sintered ceramics or thermally-crystallized glass-ceramics may be selected as starting materials for the production of aluminous keatite-containing ceramics by the above ion-exchange process. However, sintered, typically porous ceramics or glass-ceramics have previously been preferred as starting materials, particularly if free of interstitial glass, because the porous structure provides ready access to the spodumene crystals by the acid treating medium.
Glass-ceramic articles provided by the in situ crystallization of preformed glass articles are ordinarily non-porous and free of voids. Moreover, such articles often include a minor glassy phase or matrix encasing the beta-spodumene crystals which may significantly reduce the rate of ion-exchange. Thus Grossman et al. suggest, in their patent, the use of agents such as HF and NaOH to remove glassy phases prior to or during the ion-exchange treatment, in order to accelerate lithium removal by exposing the beta-spodumene crystals to the acid medium.